Traction control for automotive vehicles is well known. Traction control may use various strategies to reduce wheel slip during a wheel slip event. One approach utilizes a vehicle anti-lock braking system to detect and control wheel slip events. Engine torque may also be reduced under some conditions to reduce wheel slip. Wheel slip events in various HEV powertrains may present unique operating conditions not encountered in conventional vehicle powertrains. For example, some HEV powertrains use a single ratio gearing arrangement to connect the drive wheels to first and second electric machines that may operate primarily as a traction motor and generator, respectively. An internal combustion engine may also be coupled to the drive wheels and the electric machines. In this arrangement, the speed of the traction motor is proportional to the speed of the wheels, and the speed of the generator is proportional to wheel speed and engine speed For a given vehicle speed, the generator speed will be highest when the engine is off. Because of the relationships between wheel speed, generator speed, and engine speed, operation in all-electric drive mode with the traction motor powering the drive wheels may be limited to a vehicle speed (or wheel speed) corresponding to a maximum desired generator speed.
Wheel slip events may occur due to various operating conditions or maneuvers. For example, rough or slippery road conditions, or aggressive handling of the vehicle may result in one or more wheels slipping or leaving the ground for an extended duration, such as for 1-2 seconds or more. This causes the slipping wheel or wheels to spin faster than wheels that continue to maintain traction. When the slipping wheel regains traction, the sudden reduction in wheel speed may result in a corresponding increase in speed of one or more connected powertrain components.